Reuse of transmission resources for device to device communication

ABSTRACT

The present invention provides a method of determining a transmission power for device to device, D2D, transmissions between a first user equipment device and a second user equipment device using transmission resources being used for transmissions to a cellular network entity by a third user equipment device, the method comprising determining a measure of a path loss between the cellular network entity and the first user equipment device, and using the measure of the path loss to determine a maximum transmission power such that the D2D transmissions are received at the cellular network entity with a signal level around or below a noise level.

The present invention relates to techniques for implementingdevice-to-device (D2D) communication in a mobile communications systemin which non-orthogonal multiple access (NOMA) communication is enabled.

Current mobile communication systems like GSM, UMTS, HSPA and LTE useorthogonal radio resources to distinguish signals from and to differentusers. That means, that each user obtains a set of individual resourcesfor exclusive usage so that signals sent by different users on thesedifferent set of resources do not interfere with each other. A resourceis a certain portion of the frequency spectrum at a certain instance intime. In case of LTE, a resource may be represented by a resource blockmade up of twelve sub-carriers and seven OFDM symbols (ormultiples/fractions thereof).

In addition to conventional cellular communication between a basestation (eNB) and mobile devices (UE), 3GPP has defined a directdevice-to-device (D2D) communication mode for the LTE System. Therelated interface between two devices in D2D mode is called PC5 or“sidelink”. The resources used for D2D communication mode are sharedwith the resources for cellular communication mode. I.e. the amount ofresources and therefore the throughput in cellular communication mode isreduced in case of simultaneous use of D2D communication.

A new multiple access technology is currently studied at 3GPP within thestudy item “Downlink Multiuser Superposition Transmission for LTE”(MUST)”. One proposed solution for this study is called “Non-orthogonalMultiple Access”. The key principal is that the same resources areassigned to multiple users or multiple data streams and are usedsimultaneously. Different users or data streams are distinguished in thepower domain. The study aims to increase the cellular throughput anddoes not consider D2D communication.

Received data of each user or a data stream from the same NOMA resourceshas to be decoded successively starting with the signal received withhighest power. After that, this portion of the signal is subtracted fromthe received signal and the receiver decodes the user data or datastream received with the second highest received power from theremaining signal, and so on until all data relevant for the receiverfrom this resource are decoded. This method postulates that informationabout the different power levels used for transmission is known at thereceiver.

It is the aim of the present invention to use a modified NOMA principleto simultaneously provide cellular communication and D2D communicationon the same set of resources without limiting the cellular resources andwithout effecting the cellular system by the D2D communication, therebyproviding a full throughput in the cellular system while simultaneouslyD2D communication is provided.

In addition to documents submitted to 3GPP relating to NOMA, US WO2015/171422 A1 describes the usage of NOMA in a cellular systemincluding uplink, downlink and device-to-device transmission.Interference elimination is described for the uplink and downlink bandsbut not the suppression of interference from simultaneous operation ofthe cellular system and device-to-device communication.

Current cellular communication systems, such as 3GPP LTE, assume that(at least) two distinct resource pools exist to separate directcommunication between mobile devices from cellular communication betweenmobile device(s) and a base station. Up to now it is not specified howto enable usage of occupied cellular resources for directdevice-to-device communication (i.e. how to use the same set ofresources) without causing interferences to the cellular connection.

The known NOMA principle assumes that either a single transmitter istransmitting multiple signals with different power levels by using thesame resources, or that a single receiver is receiving multiple signalswith different power levels by using the same resources, i.e. aone-to-many or a many-to-one scenario is assumed.

US 2014/0233476 A1 describes a device to device (D2D) communicationsystem in which special sub-frames are used for D2D transmissions.Transmission power may be controlled such that a base station does notexperience signal degradation when a D2D transmission is receivedtogether with a transmission from another UE. A UE operating in a D2Dmode may receive transmit power information from the base station. Apath loss measurement between a transmitting D2D UE and a receiving D2DUE is obtained by using reference signals and this information isexchanged between the UEs.

US 2013/0310103 A1 describes a method for D2D power control in which abase station transmits power control information which a UE uses todetermine a maximum D2D transmit power.

A book entitled “Resource Management for Device-to-Device UnderlayCommunication” by Chen Xu, Lingyang Song and Zhu Han, arXiv:1311.1018v1describes a D2D scheme which relies on feedback from an eNB for transmitpower control.

US 2010/006909 A1 describes a method of uplink power control where a UEcalculates a maximum uplink transmit power from a downlink path lossmeasurement.

WO 2015/009025 A1 describes techniques for interference cancellation ina D2D system in which blind detection is used to obtain interferencesignal information which is then used to remove an interfering signalfrom a desired signal

The present invention provides a method of determining a transmissionpower for device to device, D2D, transmissions between a first userequipment device and a second user equipment device using transmissionresources being used for transmissions to a cellular network entity by athird user equipment device, the method comprising determining a measureof a path loss from the cellular network entity to the first userequipment device, and using the measure of the path loss to determine amaximum transmission power such that the D2D transmissions are receivedat the cellular network entity with a signal level around or below anoise level, wherein the determining of the maximum transmission powerincludes using a correction value for compensating for a differencebetween the measured path loss from the cellular network entity to thefirst user equipment device and a path loss from the first userequipment device to the cellular network entity.

A method that enables mobile devices with D2D capabilities to useresources for direct device-to-device (D2D) communication which arealready in use (by the same or other devices) for cellular communicationwithout disturbing the cellular connection is provided. The principlesof NOMA with interference cancellation may be applied such that no orlittle impact is caused to the cellular network and to cellular devices.

Further, the invention provides a method for a cellular network toassign identical resources several times simultaneously to differentdevices for simultaneous use for cellular transmissions and directdevice-to-device transmissions.

In a still further aspect, the invention provides a method to enhanceshort range communication receivers (e.g. Wi-Fi or Bluetooth) toeliminate interferences from LTE uplink transmissions.

The invention provides the benefit that a mobile device with D2Dcapabilities is enabled to derive a maximum allowed transmit power forthe direct D2D communication from the downlink reference signalstransmitted by the “strongest” base station (i.e. the base station thatis received with highest power). This is beneficial, as it avoids thatany interference is caused by the D2D transmission to the cellulartransmission and because no additional signalling is required.

A further benefit is that the D2D device is enabled to decode and removeinterfering signals from other mobile devices' cellular uplinktransmissions without explicit signalling about used modulation scheme,i.e. a method for blind interference cancellation is provided. This isdone by using reference signals that were already transmitted with everytransmission by the interfering device. This is beneficial, as it savescostly cellular resources, i.e. as no additional cellular resources forsuch signalling are required. Besides applying this blind interferencecancellation method to enable D2D communication, it could be used in anyreceiver to eliminate LTE uplink based interferences and therefore toincrease the reception quality and data throughput of own data. E.g.WiFi- or Bluetooth devices that operate close to an LTE band.

An eNB is enabled to select resources for D2D communication based ontheir suitability, according to one or more of the following properties:

-   -   a low order modulation scheme is assigned to the cellular uplink        transmission (e.g. QPSK and probably 16-QAM, but not 64-QAM or        256-QAM. This limitation reduces the calculation effort to        estimate each of the transmitted interfering symbol.)    -   the channel characteristic of the cellular link is (more or        less) persistent (e.g. because the respective UE is stationary).        In this case the calculated parameters for interference        cancellation could be re-used and thus effort for new        calculation is saved.    -   the transmit power used for the cellular transmission is medium        to high. This increases the reliability to correctly detect and        remove the interfering signal portion and increases the range of        the D2D transmission.

These channel persistency property qualifies stationary cellular deviceslike “relay nodes”, “cellular home network router” or “utility meters”(e.g. smart meters), to obtain cellular uplink resources that aresimultaneously used by D2D devices.

Applying the method for resource assignment is beneficial for thenetwork operator, as the cellular resources are used more efficientlyand therefore the overall system throughput and the user experience isincreased.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a communication scenario including D2D communication;

FIG. 2 illustrates a relationship between distance, reference signalpower and maximum D2D transmit power;

FIG. 3 shows an exemplary message flow chart;

FIG. 4 shows a relationship between symbols and power levels for QPSKand 16-QAM transmissions; and

FIG. 5 a division of steps between a D2D transmitter and a D2D receiver.

The following description assumes a communication system according toLTE. FIG. 1 shows an assumed scenario. However, the principles of thepresent invention shall not be restricted to LTE systems.

The following is a description of interference cancellation describingsteps for transmitting and receiving D2D-UE (or short range receiver)and eNB, respectively. The particular steps are depicted in FIG. 5 .

A first aspect of the invention, that of deriving a maximum power forD2D transmission, is illustrated by FIG. 2 .

A basic aim of the present invention is to avoid interference in theresources of the cellular system, when direct device-to-devicecommunication takes place in the same resources. The method of choice toachieve this objective is NOMA where devices or data streams can bedistinguished from one another by means of different power levels. As noadditional signalling should be introduced between UEs/D2D-UEs and thebase station, a maximum D2D transmit power has to be obtained or derivedby the transmitting D2D devices themselves. The transmit power value ischosen in such a way that interference at the cellular receiver isaround or below the typical noise level, which is not harmful for thecellular connection. It is assumed in the following embodiments, thatthe LTE uplink resources are used for D2D communication, i.e. the LTEuplink band in case of FDD or the LTE uplink sub-frames in case of TDD.The eNB is therefore the receiver of the cellular signals that are to beprotected from interference caused by D2D communication.

The maximum D2D transmit power (P_Tx_D2D_max) strongly depends on thepath loss of the D2D signals to the eNB (PL_D2D-UE_eNB) illustrated inFIG. 2 . A relationship between maximum transmit power and path loss canbe expressed as follows:

P_Tx_D2D_max=PL_D2D-UE_eNB+noise floor

In order to avoid any change to the cellular system, it is not desirableto derive this path loss directly, as it requires additional proceduresin the eNB.

Alternatively, the path loss of the D2D signals as experienced by theeNB (i.e. in uplink direction) could be estimated from the path loss ofthe opposite direction, i.e. the downlink path loss. It is almost thesame, in case the used frequency bands are identical (as in case of TDDMode). If they are different (e.g., in case the cellular system is anFDD system and D2D operates in the UL frequency band of the cellularsystem), a correction value could be applied. The correction value mightbe configurable and depend on the gap between UL and DL band on thefrequency scale (further details of this are given below).

Therefore, the transmitting D2D UE calculates the downlink path loss(PL_DL) from the reference signals transmitted by the eNB (referencesignal received power, RSRP) and the transmit power of these referencesignals (P_Ref) which is broadcasted by the eNB. It calculates themaximum D2D transmit power P_Tx_D2D_max as follows:

P_Tx_D2D_max=P_Ref−RSRP+P_correct−P_offset

Wherein:

-   -   P_Ref is the transmit power of the downlink reference signals as        indicated in the system information;    -   RSRP is the measured power of the downlink reference signals at        the D2D UE;    -   P_correct is the correction value to compensate the path loss        difference of uplink versus downlink (e.g., in an FDD system        where UL and DL bands are separated on the frequency scale by a        gap). This value could be taken from a list of values stored on        the D2D device. Different values are stored for different        uplink/downlink band constellations. Or it could be calculated        by the D2D device with the following formula:

P_correct=30*log_10(F_D2D/F_DL)

-   -   -   where F_D2D is the carrier frequency used for D2D            transmission and F_DL the Downlink Carrier Frequency;

    -   P_offset is a power offset value to ensure that the received        interference power is slightly below the noise floor.

No new measurements in the eNB are required for this calculation and nonew signalling is therefore needed.

A further aspect of the invention is that of enabling a D2D receiver toreduce interference from uplink transmissions as will now be described.

The following provides a description of a procedure designed for aD2D-receiver to eliminate interferences from LTE uplink transmissions.It could also be used in any other kind of receiver that is affected byinterferences from LTE uplink transmissions, e.g. in Wi-Fi or Bluetoothreceivers.

According to the underlying assumption of this invention, directdevice-to-device communication may simultaneously take place on the sameresources that are allocated for cellular uplink transmissions. It istherefore necessary for a proper reception of D2D related data, that thereceiving D2D terminal first reduces or eliminates interference causedby the cellular uplink transmission before decoding its own data ispossible.

Interference from cellular transmissions is not present in all cases. Itmay be, that the resource in question is currently un-used in thecellular uplink or that the UE causing interference is far enough awayfrom the D2D receiver and thus the interference power is negligible.

In one embodiment, the receiving D2D device verifies a presence ofinterference before starting interference cancellation, by measuring thereceived power in the relevant frequency spectrum. The D2D signalincludes one or more gaps in the signal to enable such interferencemeasurements. If the measured energy is above a threshold, interferenceis present. If no significant interference is detected interferencecancellation is not performed and the D2D UE starts decoding its owndata immediately.

In case interference is detected or the verification was not performed,the D2D UE starts interference cancellation. For that it is required toobtain the modulation scheme applied by the interfering UE:

In one embodiment, the receiving D2D UE applies a blind modulationscheme detection. To detect the modulation scheme, it measures persub-carrier a received interference power by evaluating demodulationreference signals (DMRS) transmitted by the interfering UE. For this itis not required to exactly know the reference symbols actually beingused, as the normalised amplitude of all used symbols is 1 and thereforeit is sufficient to measure the energy of these reference symbols. Theyare located in each slot in OFDM-Symbol #3 (Note: the symbol countstarts with 0) in case of normal cyclic prefix and #2 for extendedcyclic prefix. Then it selects the sub-carriers whose reference signalpower is above a threshold e.g. the 10 strongest subcarriers (assuming10 subcarriers are above the threshold). This ensures a low noise leveland therefore a reliable decoding of the interfering signal. The D2D-UEnormalises the signal power of each selected subcarrier, so that thereference signals of all selected subcarriers have the same amplitude.Now it verifies different hypothesises about the modulation schemes. Itcalculates the difference of the power of one assumed modulation schemeand the received data symbols (=error power). If the assumption wascorrect, the difference is zero for a time invariant channel. If thedifference is above a threshold, most likely the assumption was wrong.Therefore, the D2D UE changes to the next modulation scheme to beverified and newly calculates the error power. If it is now below athreshold, the assumed modulation scheme is correct. Otherwise, the D2DUE repeats the steps of changing the assumed modulation scheme andcalculating the error power accordingly, until it is below a threshold.This indicates the correct modulation scheme. In case of QPSK, only 1power level value exists. In case of higher order QAM Modulationschemes, more than 1 exist, e.g. 3 for 16-QAM and even more for 64-QAM,etc. In FIG. 4 the symbols for QPSK and 16-QAM and the related symbolpower levels are depicted. For calculation of the error power, thereceived symbol power is compared to all symbol power levels of theassumed modulation scheme and only the power level which is closest tothe power level of the received symbol is considered for error powercalculation.

If no modulation scheme was found, whose error power is below athreshold, the D2D UE may guess the modulation scheme. It starts withthe modulation scheme with the lowest error power. The correctness ofthe guessing is proved after interference cancellation. In case thereference signals are detected or decoding of the user data in theremaining signal is possible, the guessing was correct. Otherwiseinterference cancellation is done again with another modulation scheme.

In another embodiment the eNB indicates the used modulation scheme tothe receiving D2D UE, e.g. via broadcast or via dedicated signalling.This is the most reliable way but it is costly as it requires additionalsignalling from the eNB. The solution is advantageous in cases, wherethe applied modulation scheme is unchanged for a long time, e.g. in casethe uplink resources were assigned to a Relay Node or a device which isknown to be at a fixed position (e.g. a Router with LTE-based WAN).

In the next step, having determined the modulation scheme, the D2D UEperforms interference cancellation. For that it first calculates a phaseerror of the interfering symbols by calculating a phase difference ofthe received interference symbols compared to the nearest symbol of theassumed modulation scheme. From these individual phase error values, theaverage phase error is calculated per subcarrier over a certain timewindow (e.g one sub-frame). The length of this time window can beselected by the receiving D2D device. Longer periods will lead to a morereliable interference cancellation on the cost of a longer delay fordecoding the user data.

Then the D2D UE calculates the amplitude of the interfering symbols fromthe received DMRS. From these individual amplitude values, the averageamplitude is calculated per sub-carrier over the time window.

With the found modulation scheme, the average phase error and theaverage amplitude from the reference symbols, the D2D UE rebuilds thesignal portion of the cellular transmitter. This is done for eachsubcarrier individually, in order to better adapt to the frequencydependant phase error and attenuation. Then it removes the rebuiltsignal portion from the received overall signal.

It is to be noted that the aim of the described interferencecancellation method is to eliminate, as far as possible, the cellularportion of the received signal while keeping the D2D portion unchangedfor subsequent demodulation and decoding. To ensure that the D2D signalis not eliminated together with the cellular signal, we assume slowlychanging cellular channel properties and apply averaging over a certaintime window. As a result, per sub-carrier a single value “average phaseerror” and a single value “average amplitude” are applied to all symbolswithin the considered time window. This averaging is done because morefrequent correction, e.g. with symbol-wise values, would eliminate thephase and amplitude of the symbols containing the user data for the D2DUE.

The remaining signal is now ready for demodulation of the user data.

A further aspect of the present invention is that of providing a basestation with a mechanism for selecting suitable D2D resources.

Again, the underlying assumption of this aspect of the invention is thatthe received D2D signal is overlaid by a signal from the cellularuplink. Therefore, interference cancellation has to be performed forproper decoding of the data transmitted over the direct device-to-devicelink. To find the appropriate parameters (i.e. the applied modulationscheme for coding and phase shift and attenuation of transmissionchannel of the interfering signal) is a costly procedure regardingprocessor resources and battery power and the mobile device will benefitfrom any means to reduce the effort of interference cancellation. Themobile network can provide assistance for interference cancellation by asophisticated resource allocation scheme for D2D usage by selectingresources with good suitability for easy interference cancellation. Thisoption is shown in FIG. 5 as blocks 100 and 102 with dashed lines in thetransmitter and the receiver, respectively. The following parameterswill reduce the burden of interference cancellation:

-   -   Low order modulation scheme of the interfering UL signal:        -   This reduces the calculation effort to estimate each of the            transmitted interfering symbols.    -   Long-lasting transmission and time-invariant (persistent)        transmission channel characteristics between the originator of        the interfering signal and the receiving D2D-UE:        -   In this case the frequency of re-calculating parameters for            interference cancellation can be reduced to save computation            power.    -   High reception power of the interfering signal at the receiving        D2D UE:        -   This increases the reliability to correctly detect and            remove the interfering signal portion.

In one embodiment, enhancements are also proposed to be applied to thebase station of the cellular communication system. In case of LTE, anexemplary eNB incorporating the invention may be enabled to select andassign resources for D2D-usage that provide capabilities for easyinterference cancellation based on the above listed parameters.Therefore, the eNB is enabled to detect (almost) stationary mobiledevices (as these usually hold time-invariant transmission channelcharacteristics), that have an ongoing uplink connection and are locatedat great distance from the eNB (e.g. close to the cell edge) and/or inproximity to the D2D-UE. This often leads to a high reception power ofthe interfering signal at the D2D UE, which will ensure reliableinterference cancellation. The property of low mobility could be derivedby the eNB either by monitoring the change over time of e.g. the signalstrength or similar physical parameters in the uplink, or by obtainingthe type of device of the according cellular uplink transmitter. Ideallysuited are devices of type “Relay Node”, “cellular home network router”or “utility meters” (e.g. smart meters), as they are typically mountedand therefore unmovable. Once the eNB has selected matching devices forassignment of resources that are also assigned to D2D usage, it willensure constant (=long-lasting) resource locations (i.e. subcarriernumbers) and low modulation order of the related uplink for cellulartransmissions.

An exemplary message flow for transmission and decoding of data in casethat interference caused by transmitters in the uplink of an LTE-basedcellular communication system has to be eliminated, is depicted in FIG.3 and described in the following. For the sake of simplicity only oneD2D-Receiver (D2D-UE #2) is shown. Nevertheless more than one receivermay receive the data. Such additional receivers will perform the samesteps as D2D-UE #2. The numbering below corresponds to the numberingshown in FIG. 3 .

1. D2D-UE #1 wants to transmit a message directly (via LTE sidelink) toD2D-UE #2

2. D2D-UE #1 obtains D2D resource configurations from eNB, including

-   -   information about the nature of possible interference on the        indicated D2D resource: “No interference. D2D transmission only”        or “LTE Uplink based interference. Simultaneous D2D and cellular        transmission”    -   type of resource: “dedicated/congestion free” or        “shared/congestion based”    -   location of resources in frequency-time-grid (i.e. IDs of        subcarriers and sub-frames)    -   (optionally): applied modulation scheme of related uplink        connections.

In this example, resources are assigned, which are simultaneously usedfor cellular uplink transmissions. Consequently, interferencecancellation may be required for proper operation and D2D transmit powerlimitation have to be satisfied.

3. D2D-UE #1 derives maximum transmit power for D2D transmissionaccording to the method to derive the maximum D2D transmit powerdescribed above.

4. D2D-UE #1 transmits a message to D2D-UE #2 using a transmit powerwhich is below or around to the maximum transmit power. There may beother criteria that limit the transmit power in addition to that appliedin step 3, i.e. regulatory obligations or hardware related obligations(e.g. maximum power provided by power amplifier). D2D-UE #1 will ensurethat none of the power limits are exceeded by the used transmit power.

5. D2D-UE #2 detects cause to receive D2D-signal. E.g. because it hasreceived a corresponding paging signal previously.

6. D2D-UE #2 obtains D2D configuration data from eNB (cf. step 2 above)

7. D2D-UE #2 detects information about the nature of possibleinterference on the indicated D2D resource. As described in step 2, LTEuplink based interference is assumed. Therefore, D2D-UE #2 is removinginterferences as described above in the method to decode and removeinterfering signals. If the applied modulation scheme is included in thereceived D2D configuration data, it is used for Interferencecancellation. Otherwise, blind modulation scheme detection or guessingof the modulation scheme is applied as described above in the method todecode and remove interfering signals.

8. D2D-UE #2 decodes the data indented for this device. In case decodingwas unsuccessful, steps 7. and 8. may be repeated with a differentassumption for the modulation scheme of the interference until decodingis successful.

Further aspects of the invention include a method of performing deviceto device, D2D, communication between a first user equipment device anda second user equipment device using transmission resources being usedfor transmissions to a cellular network entity by a third user equipmentdevice, the method comprising performing a blind demodulation scheme fordemodulating transmissions by the third user equipment device in orderto perform interference cancellation to cancel signals received from thethird user equipment device prior to demodulating signals received fromthe second user equipment device.

The method may be such that the blind demodulation scheme comprisesevaluating demodulation reference signals transmitted by the third userequipment device.

A modulation scheme used by the third user equipment device may bedetermined by:

-   -   measuring a signal power of reference symbols of the reference        signals for a plurality of sub-carriers,    -   selecting a predetermined number of sub-carriers having a        reference signal power above a threshold;    -   applying an assumed demodulation scheme to the signals in these        selected sub-carriers;    -   determining whether an error power is below a threshold in which        case the related assumed demodulation scheme will be used for        demodulation. If the error power is equal or above the        threshold, a further assumed demodulation scheme is applied        until an error power is below the threshold.

These blind detection methods may be combined with the power controlaspects of the invention described above.

A further aspect of the invention provides a method of performing deviceto device, D2D, communication between a first user equipment device anda second user equipment device using transmission resources being usedfor transmissions to a cellular network entity by a third user equipmentdevice, the method comprising performing a demodulation scheme fordemodulating transmissions by the third user equipment device in orderto perform interference cancellation to cancel signals received from thethird user equipment device prior to demodulating signals received fromthe second user equipment device, the demodulation scheme having beencommunicated to the first user equipment device by the cellular networkentity.

This method may be combined with the power control aspects of theinvention described above.

A further aspect of the invention provides a method of assigning radioresources for device to device, D2D, communication between a first userequipment device and a second user equipment device, the radio resourcesbeing used for transmissions to a cellular network entity by a thirduser equipment device, wherein the radio resources are assigned for theD2D communication taking into account one or more parameters selectedfrom a list comprising:

-   -   an order of a modulation scheme of the transmissions by the        third user equipment device;    -   a measurement of transmission channel characteristics with time        between third user equipment device and a receiving user        equipment device in the D2D communication and    -   a reception power of a signal of the third user equipment device        at a receiving user equipment device in the D2D communication.

This method may be combined with the control aspects of the inventiondescribed above.

What is claimed is:
 1. A method of performing device to device, D2D,communication between a first user equipment device and a second userequipment device using transmission resources being used fortransmissions to a cellular network entity by a third user equipmentdevice, the method comprising performing a blind demodulation scheme fordemodulating transmissions by the third user equipment device in orderto perform interference cancellation to cancel signals received from thethird user equipment device prior to demodulating signals received fromthe second user equipment device.
 2. The method according to claim 1,wherein the blind demodulation scheme comprises evaluating demodulationreference signals transmitted by the third user equipment device.
 3. Themethod according to claim 2, wherein a modulation scheme used by thethird user equipment device is determined by: measuring a signal powerof reference symbols of the reference signals for a plurality ofsub-carriers, selecting a predetermined number of sub-carriers having areference signal power above a threshold; applying an assumeddemodulation scheme to the signals in these selected sub-carriers;determining whether an error power is below a threshold in which case afurther assumed demodulation scheme is applied until an error power isbelow the threshold.
 4. The method according to claim 1, furthercomprising determining a maximum transmission power for D2Dtransmissions between the first user equipment device and the seconduser equipment device using transmission resources being used fortransmissions to a cellular network entity by the third user equipmentdevice, the determining of the maximum transmission power comprising:determining a measure of a path loss between the cellular network entityand the first user equipment device, and using the measure of the pathloss to determine a maximum transmission power such that the D2Dtransmissions are received at the cellular network entity with a signallevel around or below a noise level.
 5. The method according to claim 4,wherein the determining of the maximum transmission power includes usinga correction value for compensating for a difference between themeasured path loss from the cellular network entity to the first userequipment device and a path loss from the first user equipment device tothe cellular network entity.
 6. A method of performing device to device,D2D, communication between a first user equipment device and a seconduser equipment device using transmission resources being used fortransmissions to a cellular network entity by a third user equipmentdevice, the method comprising performing a demodulation scheme fordemodulating transmissions by the third user equipment device in orderto perform interference cancellation to cancel signals received from thethird user equipment device prior to demodulating signals received fromthe second user equipment device, the demodulation scheme having beencommunicated to the first user equipment device by the cellular networkentity.
 7. The method according to claim 6, further comprisingdetermining a maximum transmission power for D2D transmissions betweenthe first user equipment device and the second user equipment deviceusing transmission resources being used for transmissions to a cellularnetwork entity by the third user equipment device, the determining ofthe maximum transmission power comprising: determining a measure of apath loss between the cellular network entity and the first userequipment device, and using the measure of the path loss to determine amaximum transmission power such that the D2D transmissions are receivedat the cellular network entity with a signal level around or below anoise level.
 8. The method according to claim 7, wherein the determiningof the maximum transmission power includes using a correction value forcompensating for a difference between the measured path loss from thecellular network entity to the first user equipment device and a pathloss from the first user equipment device to the cellular networkentity.